Robert J. Miller1,*, Henry M. Page1, Mark A. Brzezinski1,2

ABSTRACT: We investigated the extent to which temporal variation in the stable isotope composition of suspended particulate organic matter (POM) was explained by phytoplankton biomass and production at a southern California (USA) kelp forest and farther offshore in the Santa Barbara Channel. On the reef, δ13CPOM values were positively correlated with chlorophyll a concentration and phytoplankton productivity; the latter explained 62% of the variability in δ13CPOM. These relationships were weaker offshore, where variation in δ13CPOM was better explained by the abundance of dinoflagellates. As we predicted based on patterns of generally higher phytoplankton biomass and productivity along the shallow shelf, reef δ13CPOM values were typically 13C-enriched relative to values offshore. We used the relationship between chlorophyll a and reef δ13CPOM to estimate phytoplankton δ13C and the contribution of terrestrial C to coastal particulate organic carbon immediately following a rain event. These calculated terrestrial contributions explained 88% of the variability in freshwater runoff (indicated by salinity). δ15NPOM values varied across the year in association with changes in dissolved inorganic N nutrient pools. These results show that trophic studies of coastal marine ecosystems, at least off Santa Barbara, can use inshore POM stable isotope values to represent phytoplankton when freshwater runoff is low. This finding simplifies the use of stable isotopes to infer trophic relationships in southern California kelp forests. Coastal food web studies, particularly those examining kelp contributions, have typically used offshore POM isotope values to represent inshore phytoplankton. Our results show that this assumption may bias results of food web mixing models.